1. Field of the Invention
The present invention relates to a digital amplitude modulation apparatus, and more particularly to a digital amplitude modulation apparatus which uses different sampling frequencies between an analog modulating wave and a digital amplitude modulated wave.
2. Related Background Art
A digital amplitude modulation apparatus has been proposed heretofore, which digitally processes amplitude modulation such as AM, DSB and SSB.
FIG. 5 is a block diagram showing a conventional digital amplitude modulation apparatus.
An analog modulating wave S.sub.A is supplied to an A/D converter 10, sampled in response to a clock CKl (sampling frequency fsl) supplied from a timing generator 12, and converted into a digital modulating wave S.sub.D.
The digital modulating wave S.sub.D is supplied to a fold component elimination filter 14 to eliminate a fold component thereof. The filter operates in response to a clock CK2 (sampling frequency fs2) supplied from another timing generator 16.
The digital modulating wave S.sub.D, outputted from the fold compensation filter 14 is therefore associated with the sampling frequency fs2 (fsl&lt;fs2).
The digital modulating wave S.sub.D ' is inputted to an amplitude modulator 18 operating at clock CK2 (sampling frequency fs2), and multiplied by a digital carrier wave having a frequency fc to obtain a digital DSB modulated wave H.sub.D associated with the carrier wave frequency fc.
This digital DSB modulated wave H.sub.D is D-A converted into an analog DSB modulated wave H.sub.A by a D/A converter 20 which operates at clock CK2 (sampling frequency).
The reason why the sampling frequency fsl lower than fs2 for the digital DSB modulated wave H.sub.D is used by the A/D converter 10 is as follows.
In order to make it easy to eliminate the fold component of an A/D converted digital modulating signal, or to prevent generating an image signal during frequency conversion at the succeeding stage, it is preferable that the sampling frequency fs2 for the digital amplitude modulated wave HD is as high as possible.
If there is used a high sampling frequency fsl for obtaining a digital modulating wave S.sub.D, a high speed A/D converter must be used resulting in a very high cost of the apparatus.
From the above reasons, the sampling frequency fs2 is made high, whereas the sampling frequency fsl is made low.
Apart from the above, as shown in FIG. 6A, the spectrum of a digital modulating wave S.sub.D sampled at the sampling frequency fsl includes not only the original signal component A within the Nyquist bandwidth from 0 to (1/2) x fsl but also a fold component B, for example, within the bandwidth from (1/2).times.fsl to 3/2 .times.fsl. Therefore, if DSB modulation is digitally carried out without removing the fold component B, the fold distortion appears in the spectrum of the digital DSB modulated wave H.sub.D at the sampling frequency fs2 as shown in FIG. 6B, wherein both side band waves Al and A2 derived from amplitude modulation of signal A and both side band waves B1 and B2 derived from amplitude modulation of signal B are partially superposed one upon another.
It is therefore necessary to eliminate the fold component of the digital modulating wave S.sub.D prior to amplitude modulation, so that there is provided the fold component elimination filter 14 at the output of the A/D converter 10 in order to eliminate the fold component B.
The spectrum of the digital modulating wave S.sub.D ' outputted from the fold component elimination filter 14 becomes as shown in FIG. 7A, and the spectrum of the digital DSB modulated wave H.sub.D becomes as shown in FIG. 7B. Therefore, there are present only both side band waves Al and A2 derived from the DSB modulated original signal within the Nyquist bandwidth.
With the above-described conventional technique, it is necessary to provide the fold component elimination filter 14 of complicated structure between the A/D converter 10 and amplitude modulator 18, resulting in a large burden on configuring the apparatus.